The present invention relates to registration of plural image exposures formed on a photoreceptor belt by a plurality of image print bars and, more particularly, to a method and apparatus for aligning the image print bars relative to each other so that they are aligned in the scan and process directions to form registered color images In a single pass.
Image print bars used in xerographic recording systems are well known in the art. The print bar generally consists of a linear array of a plurality of discrete light emitting sources. Light emitting diode (LED) arrays are preferred for many recording applications. In order to achieve high resolution, a large number of light emitting diodes, or pixels, are arranged in a linear array and means are included for providing a relative movement between the linear array and a moving photoreceptor so as to produce a scanning movement of the linear array over the surface of the photoreceptor. Thus, the photoreceptor may be exposed to provide a desired image one line at a time as the LED array is advanced, relative to the photoreceptor either continuously or in stepping motion. Each LED in the linear array is used to expose a corresponding area in the photoreceptor to an exposure value defined by video data information applied to the drive circuits of the print bars.
In a color xerographic system, a plurality of LED print bars are positioned adjacent to the photoreceptor surface and selectively energized to create successive image exposures, one for each of the three basic colors. A fourth print bar may be added if black images are to be created as well.
FIG. 1 shows a prior art, single pass, color printing system having three exposure stations 10, 12, 14, each station including an LED print bar 10A, 12A, 14A. Each print bar is selectively addressed by video image signals processed through control circuit 15 to produce a modulated output which is coupled through a respective gradient index lens array 10B, 12B, 14B onto the surface of previously charged photoreceptor belt 16. The length of belt 16 is designed to accept an integral number of full page image areas I.sub.1 -I.sub.n, represented by dashed lines. Upstream of each exposure station are charge devices 18, 20, 22 which place a predetermined electrical charge on the surface of belt 16. As the belt moves in the indicated direction, each image area moves past each of the imaging bars, with each bar providing its own exposure pattern in response to the video data input. The exposure pattern begins when the leading edge of the image area reaches a transverse start-of-exposure line represented by a dashed arrow 23. The exposure pattern is formed of a plurality of closely spaced transverse scan lines 24 shown with exaggerated longitudinal spacing on image area 11. Downstream from each exposure station, a development system 26, 28, 30 develops a latent image of the last exposure without disturbing previously developed images. A fully developed color image Is then transferred by means not shown to an output sheet. Further details of xerographic stations in a multiple exposure single pass system are disclosed in U.S. Pat. No. 4,660,059, whose contents are hereby incorporated by reference.
With such a system as that disclosed in FIG. 1, each color image I.sub.1 -I.sub.n must be precisely aligned such that all corresponding pixels in the image areas are registered. Current requirements call for registration tolerances of approximately 125 microns (0.005 inch). The print bar alignment requirements are for the pixels of each bar to be aligned in the transverse or Y-direction of FIG. 1 as well as the process or X-direction. This alignment must be maintained through continuous revolutions (passes) of the photoreceptor.
The main cause of potential print bar misalignment is due to belt conicity in the photoreceptor belt. Belt conicity is created when the two ends of the photoreceptor sheet are welded together to form the belt, causing the two belt edges, 1 and 2 to be of slightly different lengths and imparting a conical configuration to the belt. This would create a situation, referring to FIG. 1, wherein the leading edges of images I.sub.1, I.sub.2, I.sub.3 would rotate as they translate from one position to the next, since the linear surface velocity of a point near the "base" of the belt is traveling at a greater linear surface velocity than a point near the apex. If images I.sub.2, I.sub.3 are to be perfectly registered with image I.sub.1, the leading edges must not be parallel to each other but must accommodate the rotation induced by the conicity of the belt. Since the degree and direction of the conicity of the belt varies from belt to belt, each set of print bars must be individually aligned to correct for the initial misregistration. Once the print bars are correctly aligned, other causes of misregistration, such as thermal expansion and vibration-induced wobble, must also be identified and corrected.
According to the present invention, a method and apparatus is provided for initially aligning multiple print bars in a single pass printing system, so that each bar is first aligned along the transverse or Y-axis and then along the process or X-axis, so as to compensate for the belt conicity and other registration errors. After these alignments, the images formed by each print bar will be in proper registration within the prescribed tolerances. This initial alignment is enabled by modifying the standard print bar design by adding a number of fiduciary (alignment) pixels at each end of the image bar extending outside of the image-forming, central area of the print bar. The term "fiduciary pixels" is intended to suggest that there is a predefined, positional relationship between the fiduciary pixels and the other LED pixels comprising the print bar so that the position of any pixel can be known with great accuracy. These specially designed image bars are first aligned in the transverse or Y-direction by sensing the position of a designated fiduciary pixel through an aperture formed in one edge of the belt. The rotational alignment in the X-direction, also referred to as a skew alignment, is made by sensing a corresponding fiduciary pixel at the opposite end of the image bar, through a second aperture which extends transversely and is opposite to the first aperture on the photoreceptor belt, the location of the two fiduciary pixels being a known distance apart.
Each image bar is aligned in the same manner. Following initial alignment, further alignment is accomplished by monitoring the position of the two previously designated fiduciary pixels and modifying the bar position in response to detection of misregistration signals. More particularly, the present invention relates to an imaging system for forming multiple image exposure frames on a photoconductive member during a single pass including:
a photoreceptor belt adapted to accommodate the formation of an integral number of image exposure frames, said belt having a first and second alignment aperture on opposite sides of the belt width and outside of the exposure frame,
a plurality of linear image print bars, each print bar associated with the formation of one of said image exposure frames, each print bar having a central portion of light emitting pixels which are selectively activated to form said image exposure areas and first and second end portions of light emitting pixels outside of said exposure area which are selectively activated for print bar alignment and registration purposes,
a first and second detecting means associated with said first and second end portions, respectively, for detecting the position of selected ones of said first and second end pixel end portions when said end portions are visible through said alignment aperture, and
means for moving each of said print bars in a transverse direction to establish a transverse registration for each bar and means for rotating the print bar until the detected position signals from said first and second detecting means are concurrent.